[fix] models being reguarded when they are already unguarded

[fabio-ivona]

This PR will fix this code

        $model->unguard();
        $model->fill(unserialize($modelData));
        $model->exists = true;
        $model->reguard();

that reguards all models even if Model::unguard() has been called in a service provider

with the proposed fix, this won't happen anymore